Gamma Ray Bursts Spawned by New Black Holes?

SAN DIEGO--A supernova in a galaxy 140 million light-years away might turn out to be the Rosetta stone of gamma ray bursts (GRBs), mysterious cataclysms that unleash staggering amounts of energy. As discussed here last week at a meeting of the American Astronomical Society, the stellar explosion took place at about the same time and the same point in the sky as a 25 April GRB, marking the first time that a burst has been linked with anything more specific than a bright spot in the sky. If the link between the two events holds up, it could favor the idea that the collapse of massive stars--a process akin to the one that triggers many supernovae--sparks these outpourings of gamma rays.

Over the last year, the search for a GRB mechanism has heated up as astronomers have realized that many bursts occur in the far reaches of the observable universe, implying that they are unimaginably powerful. In one scenario, the bursts are triggered when two supercompact neutron stars (or a neutron star and a black hole) collide and merge. Another theory, developed by Stan Woosley of the University of California, Santa Cruz, and Bohdan Paczynski of Princeton University, attributes them instead to the sudden collapse of a very massive star--what Paczynski calls a hypernova. In many supernovae, the stellar implosion stops when the core of the star has collapsed into a neutron star. But in a hypernova, the progenitor star is so massive that the collapse continues all the way to a black hole. Matter vanishing down the gullet of the black hole emits a final surge of gamma rays--the gamma ray burst--and triggers a shock wave. The shock wave heats the sparse matter near the star, creating a supernovalike display.

The 25 April burst could tip the scale towards the hypernova mechanism. After the Italian-Dutch BeppoSAX satellite pinpointed the burst's position, Titus Galama and Paul Vreeswijk of the University of Amsterdam pointed an optical telescope at the spot and found a supernova at the same position, as they describe in a paper submitted to Nature. Woosley argues that the two events are almost certainly related. "This could well be the missing link" that astronomers have been looking for to explain GRBs, he says.

Some experts won't go so far. "The connection [between the supernova and the gamma ray burst] is unclear," says Ralph Wijers of the State University of New York, Stony Brook. Other astronomers say that even if the 25 April burst did come from the supernova, which exploded in a relatively nearby galaxy, bursts at much greater distances might need a different mechanism that can pump out thousands of times more energy.

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